1. Field of the Invention
The present invention relates to a process for preparing biaxially oriented hollow shaped articles from thermoplastic materials. The invention also relates to hollow shaped articles with improved thermomechanical properties, which are obtained according to the process.
2. Description of the Prior Art
The production of hollow biaxially oriented shaped articles uses semi-finished products which are obtained in a previous or intermediate stage of production and which may be sheets, plates, discs, or various flat articles (in case of thermoforming), parisons or preforms (in the case of biaxially oriented drawing of flasks, bottles, and the like).
Two methods in particular are known for the manufacture of biaxially oriented bottles from plastic materials. The first method, the so called "cold parison" method, comprises manufacturing a preform, cooling it to ambient temperature, storing it, reheating the preform to a temperature wherein the material exhibits a visco-elasticity which is suitable for biaxial orientation of the material, and then forming the shaped article by drawing and blowing in a cooled mold, in order to obtain and maintain the final hollow shape of the article. The second method, the so called "in line" or "hot parison" method, comprises for example injection- and blow-molding, injection- and blow-molding with biaxial orientation, and extrusion blow-molding with biaxial orientation, by forming a parison by injection, injection-blowing or extrusion-blowing. After the parison has been cooled to a sufficient degree, in order to bring it to the desired orientation temperature, it is drawn and blown while in a mold which is suitable for molding under cooling. If desired, in addition to this treatment, a step of conditioning the preform or parison under temperature, or providing a homogenous distribution of the temperature in the preform or the parison, may be added.
The molecular orientation of the material improves the transparency and the gloss of the hollow shaped articles and provides them with an increased rigidity, an improved resistance against scratching and shocks, a reduced permeability for gases, and a reduced tendency for creepflow. The improvement of the mechanical and optical properties which is observed in numerous oriented thermoplastic polymers is particularly important and advantageous in the case of hollow shaped articles, in particular articles made from semicrystalline polymers, especially those polymers which can be obtained in amorphous form as a preform and which crystallize to large degree under orientation in the course of the biaxial drawing, thereby preserving their transparency. Ethyleneglycol polyterephthalate (PET) is a typical example in this regard.
However, the biaxial orientation of the hollow shaped article has the disadvantage of strongly increasing internal stresses, thereby causing a reduction of dimensional stability under heat. This effect is due to the fact that the oriented material has a tendency to shrink, for example during hot filling, which results in release of the stresses, causing distortion and deformation of the hollow shaped article, e.g., a container, which in the case of the oriented material can take place at a lower temperature than in the case of a nonoriented material. This phenomenon is particularly important in the case of polymers drawn from an amorphous preform which undergo strong crystallization during drawing, such as polyesters and particularly PET.
During the blow-molding of objects with an irregular shape, such as bottles, from a regularly shaped preform, the stresses are not of an equal degree in all parts of the finished object. For example, the bottom and the neck of the bottle are drawn little with respect to the median part. This results in the fact that not all of the parts have the same degree of crystallinity, as certain parts may remain in the amorphous state.
In the amorphous state the material is transparent. In the crystalline state, the material is either opaque (spherullitic crystallization) or transparent (nonspherullitic crystallization) depending on the conditions of temperature and the degree of molecule orientation. For PET, rapid spherullitic crystallization of the nonoriented, amorphous material is observed beginning at 140.degree. C. The result is an opacification of parts, such as the zones close to the neck and the bottom, while the bioriented parts are transparent (nonspherullitic crystalline state).
It is well known that the rigidity of a hollow shaped article is dependent on the intrinsic rigidity of the material (modulus of rigidity), and also on the degree of orientation, the shape and thickness of the wall (mean thickness and regularity of the walls). In flasks in which the wall is relatively thin, ribs are incorporated to increase the rigidity of the walls, in particular against transversely applied compression stresses. Furthermore, the rigidity depends on the form and the deepness of the ribs. Thus, the reproduction of the ribs of the mold (stamped reproduction) is not very satisfactory in the case of hollow shaped biaxially oriented articles which are blow-molded on a cooled mold, even in the case of blow-molding under high pressure. Therefore, in spite of their higher modulus of elasticity, biaxially oriented shaped articles with ribbed walls generally do not exhibit better mechanical properties under instant compression than do nonoriented hollow shaped articles.
Consequently, biaxially oriented hollow shaped articles, which have been developed for use as containers for pressurized liquids, usually are in the form of containers having smooth walls. In many cases a lowering of the dimensional stability due to the presence of internal stress renders the hollow biaxially oriented shaped article unsuitable for filling with hot liquids. Also, such materials are not suitable for the manufacture of hollow shaped articles which later have to undergo a pasteurization, or reusable containers which have to be washable under heat.
In order to increase the dimensional stability of biaxially oriented hollow shaped articles, it has already been proposed to subject these articles to a thermo-fixing or heat setting treatment in such a way that all of the remaining internal stresses and compulsions are eliminated. This total thermofixation treatment is closely copied from the technique which is used in the field of film and fiber making, and is effected at high temperature, after the article has obtained its final shape. For example, French Pat. No. 2,285,978 discloses a thermo-fixing treatment after blow-molding, which in the case of crystallizable thermoplastic materials is effected at a temperature of the material above 140.degree. C. For PET, prior art heat setting temperatures range from 140.degree.-210.degree. C., for example. This thermo-fixing treatment at an elevated temperature results in an increase in crystallinity as a function of temperature and treatment time, and includes numerous disadvantages, some of which are:
a decrease in the rate of production which becomes marked as the difference between the thermo-fixing temperature and the temperature at which the article is taken from the mold increases; PA1 a risk that substantial distortions and contractions in various parts of the shaped article may occur during removal from the mold, if the thermo-fixing period is not sufficiently long; PA1 the disadvantages which are inherently connected with heating metallic molds to very high temperatures and maintaining them at these temperatures, such as dilatation problems, consumption of energy, and the like; PA1 the risk of spherulitic crystallization and connected therewith a loss of transparency in the lesser stretched portions of the article. PA1 (a) introducing the thermoplastic material, in the shape of a hollow preform, a hollow parison, or a sheet, into the mold cavity of a mold adapted for blow-molding under heat; PA1 (b) heating the thermoplastic material to about its temperature of biaxial orientation; PA1 (c) bringing the thermoplastic material into close contact with the walls of the mold by applying an internal pressure to the interior of the article within the mold cavity; PA1 (d) maintaining the thermoplastic material in close contact with the walls of the mold at a temperature which is suitable for molding and which is in the range from about the minimum effective temperature for biaxial orientation of the thermoplastic material to about 40.degree. C. above the minimum effective temperature for biaxial orientation of the thermoplastic material for a period of time which is sufficient to form a partially thermo-stabilized hollow shaped article within the mold; PA1 (e) allowing the hollow shaped article to cool; and PA1 (f) removing the hollow shaped article from the mold.